1. Field of the Invention
The presently disclosed subject matter relates to an optical tomographic imaging apparatus, an interference signal processing method, and an endoscope apparatus. More particularly, the presently disclosed subject matter relates to an optical tomographic imaging apparatus configured to generates an optical tomographic image by OCT (Optical Coherence Tomography) measurement, an interference signal processing method used in the optical tomographic imaging apparatus, and an endoscope apparatus used together with the optical tomographic imaging apparatus.
2. Description of the Related Art
Conventionally, it has been proposed that, when an optical tomographic image of a living tissue is obtained, an optical tomographic image obtaining apparatus using OCT measurement is used. In addition to the case where tomographic images of the ocular fundus, the anterior ocular segment, and the skin are obtained, the optical tomographic image obtaining apparatus is applied to observation of various parts, such as observation of an arterial vascular wall performed by using an OCT probe (optical probe), and observation of a digestive organ performed by inserting the OCT probe through a forceps channel of an endoscope. In the optical tomographic image obtaining apparatus, after a low coherent light beam emitted from a light source is divided into a measuring light beam and a reference light beam, a light beam, which is reflected or back scattered by a measuring object at the time when the measuring light beam is irradiated onto the measuring object, is multiplexed with the reference light beam, and an optical tomographic image is obtained based on the intensity of the interference light beam between the reflected light beam and the reference light beam. In the following, the reflected light beam and the back scattered light beam from the measuring object are collectively expressed as a reflected light beam.
The above described OCT measurement is roughly classified into the TD-OCT (Time domain OCT) measurement and the FD-OCT (Fourier Domain OCT) measurement. The TD-OCT (Time domain OCT) measurement is a method that, while changing the optical path length of a reference light beam, measures the intensity of an interference light beam, and thereby obtains the intensity distribution of reflected light beams which corresponds to positions in the depth direction (hereinafter referred to as depth positions) of a measuring object.
On the other hand, the FD-OCT (Fourier Domain OCT) measurement is a method that, without changing the optical path lengths of a reference light beam and a signal light beam, measures the intensity of an interference light beam for each spectral component of the light beam, and performs, by using a computer, frequency analysis, as represented by Fourier transform, for the spectral interference intensity signals obtained here, so as to thereby obtain the intensity distribution of reflected light beams which corresponds to the depth positions of a measuring object. In recent years, the FD-OCT measurement has been attracting attention as a method for enabling high-speed measurement, because the mechanical scanning needed for the TD-OCT is not necessary in the FD-OCT measurement. As typical apparatus configurations for performing the FD (Fourier Domain)-OCT measurement, it is possible to list two types of apparatuses based on SD-OCT (Spectral Domain OCT) and SS-OCT (Swept source OCT).
The OCT measuring technique is highly expected to be applied, in particular, to the optical biopsy (biopsy) in which a lesion region is located by non-invasive cell level measurement. However, the resolution of the OCT is about 10 μm at present, and is insufficient for the observation at the level of cell size of 10 to 20 μm. Thus, it is desired to improve the resolution of the OCT.
The axial resolution of OCT:Δz is determined by the following expression (1) using the spectral width and the center wavelength of a light source used in the OCT.Δz=(2·ln 2/π)·(λ2/Δλ)  (1)
where Δλ is the spectral width, and λ is the center wavelength.
In order to improve the resolution of OCT, it is necessary to expand the spectral width of the light source. Thus, research and development of a Ti:sapphire light source, and the like, have been promoted. However, these wide band light sources at present have disadvantages that their cost is high and that they are not suitable for mass production.
In order to solve these problems, in recent years, it has been investigated to improve the resolution of OCT in such a way that a wide band light source is produced in a pseudo manner by using SLDs having a plurality of wavelength bands. However, this method for increasing the band width of the light source in the pseudo manner has a problem that the SLD light sources, which are generally used for diagnosing a living body and which have the center wavelengths of 1.0 μm and the 1.3 μm, cannot cover all the spectral bands and hence the spectral bands are separated from each other. Thus, an interference signal is discontinuously divided due to the separation between spectral bands, so that an artifact is caused in an OCT image. In particular, a signal obtained in the TD system becomes an OCT image itself, and hence it is difficult to prevent the generation of the artifact.
Thus, multiplexing OCT techniques using the FD system are disclosed, for example, in the following patent documents.
Japanese Patent Application Laid-Open No. 2008-128708 discloses a multiplexing OCT technique which combines a plurality of multiplexing OCT systems using a plurality of light sources.
Japanese Patent Application Laid-Open No. 2008-261768 discloses a multiplexing OCT technique which uses an average value and a weighted average value to estimate, from an interference signal obtained from a spectrum having spectral bands separated from each other, an interference signal at the central part of the spectrum.
Japanese Patent Application Laid-Open No. 2006-47264 discloses a multiplexing OCT technique by which the output of a variable wavelength light source in a multiplexing OCT system using a plurality of light sources is monitored for multiplication correction.